Abstract: A mobile jack for raising and lowering a floating storage tank roof. The mobile jack includes a fluid-actuated cylinder and ram housed in a jack structure having multiple telescoping sections. The jack is designed to slide onto the forks of equipment to transport the jack to a desired location beneath the roof in preparation for lifting the roof and can be clamped to retain the jack in place during transport. The cylinder and ram are utilized to extend and retract a portion of the jack structure to raise and lower the roof of the floating roof storage tank.